The present invention relates to improvements in or relating to electronic components. More particularly, the present invention relates to improvements in or relating to capacitors, in particular, capacitor electrodes.
The advancement of power semiconductor devices has resulted in reductions in manufacturing costs, component count and maintenance. This has resulted in the production of cheaper, more compact, lighter and quieter electronic equipment.
Over the last 30 years, traction and power transmission systems have seized upon these advances and consequently, the mass and volume of traction and power systems have also been significantly reduced.
As the result of continued evolution of power electronic systems, capacitor manufacturers have had to reduce the cost, mass and the volume of capacitor components. The impetus behind capacitor evolution has been to follow the positive advances of other electronic components, that is, with a view to reducing the final unit cost thereof. This has generally been reflected by the increase in energy density (J/liter) of capacitors. In recent years, the limits of capacitor design have been extended to enable the thin polymeric films of the capacitor electrodes, having a thickness of 1-30 xcexcm, to be subjected to  greater than 200 V/xcexcm at elevated temperatures, for lifetimes of 20 years or more, giving energy densities in excess of 200 J/liter.
With a view to reducing the cost, mass and volume of such capacitor components, it is known to utilise specific independent techniques in order to increase the voltage stress capability of capacitors (Voltage stress being expressed as volts per micron (V/xcexc)). For example, it is known to increase the limits of plain metallised film capacitors by the independent application of either gradation of the metallised surface of the capacitor electrode, or by utilising segmentation techniques. The limits of utilising such known techniques have now been reached.
Therefore, there is a need for a new technique in which to achieve a capacitor design with greater voltage stress capability than previously known.
According to a first aspect of the present invention there is provided a capacitor electrode comprising a film base member upon which is located connection means so that the capacitor electrode may be connected to an external component, and connected to the connection means a segmented metallised layer having metallised segments interconnected by current gates, characterised in that the segmented metallised layer is gradated.
It is an object of the present invention to provide an improved capacitor electrode. In particular, and since the segmented metallised layer is gradated, i.e. since the metallised layer is deposited such that the thickness of the segmented metallised layer varies along the length of the capacitor electrode, the voltage stress capability of a capacitor in accordance with the present invention is surprisingly greater than that obtainable by currently utilised techniques.
It is believed to be worthy of note that in the active technical field of capacitor design, no one has considered, or disclosed, the idea of gradating the segmented metallised layers present on such capacitor electrodes, that is, with a view to increasing the voltage stress capability of a capacitor.
The combination of such techniques enables two levels of electrical breakdown protection to be implemented into the capacitor electrode design. The first level of protection is achieved by the gradation, which will reduce the amount of the active capacitor area lost for each small-scale self-healing operation, namely, by burning off a portion of the metallised layer such that the weak point on the capacitor electrode is isolated. The second level of electrical protection concerns larger electrical faults that are protected against by splitting the metallisation surface into a specific pattern or segments, interconnected by current or fuse gates. Such patterns are generically referred to as xe2x80x9cT-notchxe2x80x9d and xe2x80x9cMosaicxe2x80x9d. In the event of a larger electrical fault, the fuse gates will bum off resulting in the isolation of a complete segment.
In a preferred embodiment, the width of the current gates increases as the thickness of the segmented metallised layer decreases. Preferably, the width of the current gates increases in such a manner that the cross sectional area of the all the current gates on a capacitor electrode are constant or substantially constant. This has the advantage in that the current gates located in the thinner regions of the segmented metallised layer will be less prone to burning off in the event of a small-scale self-healing operation and hence, the isolation of a complete segment, and the large capacitance loss associated therewith, is avoided.
In a preferred embodiment, the segmented metallised layer has a gradated profile as shown in any one of FIGS. 1a, 1b, 1c, 1d and 4.
Further preferably, the film base member comprises biaxially orientated polypropylene.
Preferably, the segmented metallised layer nearest the connection means is maintained at a resistivity of 2-5 xcexa9/square.
Further preferably, the segmented metallised layer comprises aluminium and/or zinc in variable concentrations, plus trace pure metals.
In a further aspect of the present invention there is provided a process for producing a capacitor electrode comprising the steps of:
providing a surface of a film base member with a masking pattern, placing the film base member in a partial vacuum;
passing the film base member by a member provided with a slit; and
evaporating metal from the slit to form a metallic cloud such that a metal layer is deposited onto the film base member such that the segmentation of the deposited metal layer is determined by the masking pattern and the gradation of the deposited metal layer is determined by the shape of the slit .
In another aspect of the present invention there is provided a capacitor including a capacitor electrode in accordance with the present invention.